1. Field of the Invention
This invention relates to a data compression apparatus for a video signal and a method of data compression for a video signal. The invention relates particularly to a data compression apparatus for compressing data of still color picture images or color moving image in order to record or transmit the data and to a method of data compressing of the still color picture images or the color moving image in order to record or transmit the data.
2. Description of the Prior Art
Data compression is known which is used for effective recording when image data is recorded on a recording medium such as a magnetic disc, etc. For example, the image data is divided into blocks, each having N.times.N pixels and their image data of each block is orthogonal-transformed. The orthogonally transformed data is quantized through given steps. Then, it is subjected to zero-run-length coding or Huffman coding. In such data compression, data is compressed at a high efficiency. However, an amount of coded data is dependent of the image.
Therefore, formerly, the amount of coded data is controlled to remain constant as follows:
A first method is that the amount of actually quantized data through a given quantizing step size is obtained by operation, the quantization is performed again with the number of quantization steps (the quantizing step size) so changed that the amount of data becomes a desired value in accordance with the result of the operation.
A second method is based on that there is a given relation between coefficients of data after the orthogonal transformation and the amount of coded data and is as follows:
Sum of squares of coefficients at each clock is obtained by operation. Each block is classified into four classes, for example, in accordance with the obtained sum. Then, many bits are allotted to a block whose class is of a large amount of data and a few bits are allotted to a block whose class is of a small amount of data.
However, in the above-mentioned first method, high speed processing is difficult because processing of calculation of the amount of actually quantized data should be repeated at least twice.
In the above-mentioned second method, complicated processing is necessary because orthogonal transform processing as well as addition of information indicative of class to data is required, so that the amount of coded data becomes large.